Field of the Invention
The present invention relates to a wavelength tunable light source and an OCT (Optical Coherence Tomography) apparatus using the wavelength tunable light source. In particular, the present invention relates to a wavelength tunable light source and the like capable of varying oscillation wavelength by moving a reflecting mirror using, for example, a MEMS (Micro-Electromechanical System) mechanism as a wavelength tuning mechanism.
Description of the Related Art
In recent years, in the field of medical diagnostic imaging, a noninvasive, contactless, diagnostic method using near-infrared light has been studied, and the method is referred to as optical coherence tomography (OCT). Several OCT techniques are being studied, and OCT which uses a swept frequency source (Swept Source Optical Coherence Tomography: hereinafter referred to as SS-OCT), in particular, is drawing attention. A wide tunable range, narrow spectral line width, and high-speed operation are required of wavelength tunable light sources for SS-OCT. Specifically, a configuration which varies the oscillation wavelength by operating a vertical cavity surface emitting laser (hereinafter referred to as VCSEL) using a MEMS-based electrostatic actuator is disclosed by T. Yano et al. in “Wavelength Modulation Over 500 kHz of Micromechanically Tunable InP-Based VCSELs With Si-MEMS Technology,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 15, No. 3, May/June 2009.
A configuration of a moving part of the wavelength tunable light source disclosed by T. Yano et al. is shown in FIG. 5A and FIG. 5B, where the moving part is driven by the electrostatic actuator. FIG. 5A is a top view and FIG. 5B is a sectional view taken along line 5B-5B in FIG. 5A. The wavelength tunable light source includes a moving part shown in FIG. 5A and FIG. 5B and a VCSEL substrate (not shown), and a wavelength-tuning operation is performed by varying spacing between a fixed mirror (not shown) on the side of the VCSEL substrate and a movable mirror 406 on the side of the moving part. The moving part driven by the electrostatic actuator is constructed from an SOI (Silicon-On-Insulator) substrate 414 and includes an Si substrate 400, an SiO2 layer 402, and an Si layer 404 which is a beam material. The movable mirror 406 which is a dielectric layer is formed on a beam 412 made of the Si layer 404. When a voltage is applied to the Si substrate 400 and beam 412, electrostatic attraction acts through a gap 410 between the Si substrate 400 and beam 412, and then the Si beam 412 undergoes flexural deformation by being attracted to the Si substrate 400. Consequently, the movable mirror 406 is attracted toward the Si substrate 400 as well, changing the spacing from the fixed mirror (not shown) and thereby changing the oscillation wavelength of the VCSEL. Oscillating light is emitted through a light-extracting hole 408 formed in the Si substrate 400.
However, the moving part with the above configuration needs a high drive voltage ranging from 91 V to 151 V. With the technique disclosed by T. Yano et al., since the moving part is constructed from SOI while SiO2 (the SiO2 layer 402) with a high breakdown voltage is used for an insulating layer, a high drive voltage can be used for a driving voltage. However, in order to construct the moving part integrally with the VCSEL substrate, it is common practice to use a compound semiconductor (e.g., GaAs) highly consistent with the substrate, for the insulating layer, but this results in a breakdown voltage an order of magnitude lower than SiO2. Consequently, the breakdown voltage cannot be increased.
A common technique used to reduce the drive voltage involves reducing the stiffness of the beam by increasing the length of the beam or using a material with a low Young's Modulus for the beam, but reduced stiffness of the beam can result in a reduced resonance frequency. In view of the above problem, an object of the present invention is to provide a wavelength tunable light source which allows a drive voltage to be reduced without reducing operation speed as well as to provide an apparatus which uses the wavelength tunable light source.